leetcode二叉树最大深度的俩种解法

/**
 * <p>给定一个二叉树，找出其最大深度。</p>
 *
 * <p>二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。</p>
 *
 * <p><strong>说明:</strong>&nbsp;叶子节点是指没有子节点的节点。</p>
 *
 * <p><strong>示例：</strong><br>
 * 给定二叉树 <code>[3,9,20,null,null,15,7]</code>，</p>
 *
 * <pre>    3
 * / \
 * 9  20
 * /  \
 * 15   7</pre>
 *
 * <p>返回它的最大深度&nbsp;3 。</p>
 * <div><div>Related Topics</div><div><li>树</li><li>深度优先搜索</li><li>广度优先搜索</li><li>二叉树</li></div></div><br><div><li> 1267</li><li> 0</li></div>
 */

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {

    int res = 0;
    int depth = 0;

    //解法2
    public int maxDepth(TreeNode root) {
         if(root == null){
             return 0;
         }

         int a = maxDepth(root.left);
         int b = maxDepth(root.right);
         //每次遍历要算上根的高度
         int res = Math.max(a,b)+1;
         return res;
    }

    //解法1 后序遍历
    public int maxDepth(TreeNode root) {
        traverse(root);
        return res;
    }

    void traverse(TreeNode root) {
        if (root == null) {
            res = Math.max(res, depth);
            return;
        }
        depth++;
        traverse(root.left);
        traverse(root.right);
        depth--;
    }


}
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